Open-cell polypropylene particle foams

ABSTRACT

The invention relates to foam beads of propylene polymers which comprise more than 40% of open cells. They are produced by impregnating propylene polymer minigranules which comprise from 1 to 40% by weight of a cell opener with a volatile blowing agent in aqueous suspension and subsequently decompressing the mixture.

The invention relates to open-celled foam beads based on propylenepolymers.

Foam parts made from propylene polymers are increasingly being employedfor parts in the interior of automobiles, as packaging materials and forleisure articles. They are obtained by post-expansion and sintering ofcorresponding foam beads. These are in turn produced by known processes(for example EP-A 95109) by impregnation of polypropylene granules inaqueous suspension with a volatile blowing agent in a pressurecontainer, followed by decompression of the mixture. Foam beads producedin this way exhibit a so-called double peak in the DSC thermodiagram,i.e. at least one high-temperature peak at a higher temperature than themelting peak of the propylene polymer employed (see, for example, EP-A415744). They are distinguished by particularly good processingproperties and good mechanical properties, for example heat distortionresistance. Foam beads of this type normally have closed cells; forexample, the foam beads according to EP-A 95109 have at least 65% ofclosed cells. This is also desired in most applications.

EP-A 1016690 describes foam beads made from propylene polymers whichcomprise water as blowing agent and from 0.05 to 10% by weight of ahydrophilic polymer, for example a polyamide, a polyester or polyvinylalcohol. However, they comprise at least 85% of closed cells, i.e. arenot open-celled.

Open-celled plastic foams are generally more elastic than closed-cellfoams and have a sound-insulating action, which is advantageous, forexample, for automobile interior trim and in some packaging materials.WO 00/15700 describes open-celled polyolefin foams having goodsound-absorbing properties. They are boards, films or pipes produced byextrusion of a polyolefin melt together with a volatile organic blowingagent. In this foam extrusion, it is only possible to produce moldingshaving a simple geometry. In addition, the foams do not exhibit theabove-described double peak in the DSC thermodiagram, and consequently,for example, their heat distortion resistance is inadequate for someapplications.

It is an object of the present invention to provide open-celled foambeads based on propylene polymers which exhibit, in particular, goodmechanical properties and can be further processed in a variety of ways.

We have found that this object is achieved by open-celled foam beadshaving a mean bead size of from 1 to 10 mm and a bulk density of from 5to 200 g/l based on propylene polymers and having a proportion of opencells (in accordance with DIN ISO 4590) of greater than 40%.

A preferred process for the production of open-celled foam beads of thistype comprises impregnating propylene polymer beads in aqueoussuspension with a volatile blowing agent in a pressure container atelevated temperature, and subsequently decompressing the mixture. Thepropylene polymer beads comprise from 1 to 40% by weight of a cellopener, for example a polar, water-insoluble thermoplastic, awater-soluble polymer or a needle-shaped inorganic solid.

For the purposes of the present invention, propylene polymers are:

-   a. homopolypropylene,-   b. random copolymers of propylene with up to 15% by weight of    ethylene and/or 1-butene, preferably copolymers of propylene with    0.5 to 6% of ethylene or with from 0.5 to 12% by weight of 1-butene,    or terpolymers of propylene, from 0.5 to 6% by weight of ethylene    and from 0.5 to 6% by weight of 1-butene,-   c. mixtures of a. or b. with from 0.1 to 75% by weight, preferably    from 3 to 50% by weight, of a polyolefin elastomer, for example an    ethylene-propylene block copolymer.

Propylene polymers produced either with Ziegler catalysts or withmetallocene catalysts are suitable.

The crystalline melting point (DSC peak maximum) of the propylenepolymers is between 120 and 170° C. Their enthalpy of melting,determined by the DSC method, is preferably between 50 and 110 J/g, andtheir melt flow index (MFI) (230° C., 2.16 kp) in accordance with DIN53735 is between 2 and 25 g/10 min.

The open-celled foam beads according to the invention are producedstarting from granules, which preferably have mean diameters of from 0.2to 10 mm, in particular from 0.5 to 5 mm. These usually cylindrical orspherical minigranules are produced by extruding the propylene polymertogether with the admixed cell opener and further additives, forcing themixture out of the extruder, if desired cooling the mixture, andgranulating the mixture.

It is essential that the minigranules comprise from 1 to 40% by weight,preferably from 5 to 35% by weight, of a cell opener. Preferred cellopeners are polar, water-insoluble thermoplastics, preferably apolyamide (PA) or polyoxymethylene (POM); water-soluble polymers,preferably polyvinylpyrrolidone (PVP), polyvinyl acetate andpolyethylene oxide; furthermore needle-shaped inorganic solids,preferably cut glass having a length of from 0.25 to 5 mm. In addition,the minigranules may comprise conventional additives, such asantioxidants, stabilizers, flame retardants, waxes, pigments, dyes andin particular nucleating agents, such as talc, graphite powder,pyrogenic silicic acids, zeolites, bentonites and polyolefin waxes.

The minigranules are dispersed in a suspension medium in a stirredreactor. The preferred suspension medium is water. In this case,suspension assistants have to be added in order to ensure uniformdistribution of the minigranules in the suspension medium. Suitablesuspension assistants are water-insoluble inorganic stabilizers, such astricalcium phosphate, magnesium pyrophosphate and calcium carbonate;furthermore polyvinyl alcohol and surfactants, such as aryl sulfonatesand fatty alcohol oxylates. They are employed in amounts of from 0.05 to5% by weight, based on the minigranules. A volatile blowing agent isadded to the suspension in amounts of from 2 to 50% by weight,preferably from 5 to 30% by weight, based on the minigranules. Theblowing agent can be added before, during or after heating of thereactor contents. Suitable blowing agents are organic compounds having aboiling point of between −5 and 150° C., for example alkanes, ketonesand ethers, as well as inorganic gases, such as nitrogen and carbondioxide. Particular preference is given to C₄- to C₆-hydrocarbons.

During impregnation, the temperature should be in the vicinity of thesoftening point of the propylene polymer. It can be from 40° C. below to15° C. above the melting point (crystalline melting point). Preferredimpregnation temperatures are between 120° C. and 160° C. Depending onthe type and amount of blowing agent and depending on the temperaturelevel, a pressure which is generally greater than 2 bar and does notexceed 40 bar becomes established in the reactor. The impregnation timesshould generally be between 0.5 and 10 hours, after which the reactor isdecompressed quickly. During this operation, the propylene polymer beadsimpregnated with blowing agent expand. Due to the cell opener, theopen-celled foam beads according to the invention form in the process.After decompression and discharge from the reactor, the foam beads areseparated from the suspension medium and washed. If a water-soluble cellopener was employed, this is substantially washed out at the same time.In the case of water-insoluble cell openers, these remain present in thefoam beads.

The open-celled foam beads according to the invention have particularlygood mechanical properties if they have, in the DSC melting diagram, atleast one high-temperature peak at a higher temperature than the meltingpeak of the propylene polymer employed. The mean bead size of the foambeads is from 1 to 10 mm, preferably from 2 to 8 mm; their bulk densitycan be between 5 and 200 g/l, preferably between 10 and 100 g/l. Theyare characterized by a proportion of open cells (in accordance with DINISO 4590) of greater than 40%, preferably greater than 50% and inparticular greater than 75%. In the ideal case, virtually all cells areopen, i.e. the foam structure consists only of cell webs. The mean celldiameter is preferably from 0.01 to 0.5 mm, in particular from 0.05 to0.3 mm.

Foam moldings can be produced from the foam beads according to theinvention by known processes comprising post-expansion and sintering.

The parts (P) mentioned in the examples are by weight.

EXAMPLES

P below denotes parts by weight.

Starting Materials:

-   -   PP1: Novolen 3200 MC; polypropylene from Basell GmbH    -   POM1: Ultraform N 2320 (BASF AG)    -   PA1: Ultramid B 3 (BASF AG)    -   PS1: standard polystyrene having M_(w)=195,000 g/mol, MVR=10        cm³/10 min (BASF AG)    -   PVP1: polyvinylpyrrolidone, Kollidon K 30 from BASF AG    -   Glass: cut glass 3786    -   Wax1: Luwax AF 31; polyethylene (Mn 3000) from BASF AG    -   Talc: HP 325    -   s-Pentane: technical-grade pentane isomer mixture    -   n/i-Butane: technical-grade butane isomer mixture    -   Antiblock1: CaCO₃;    -   Antiblock2: tricalcium phosphate    -   Surf1: fatty alcohol oxylate    -   Surf2: aryl sulfonate

Production of the Granules:

In order to produce the granules, the starting materials were mixed, fedto the extruder and converted into minigranules (d: 0.8-1.2 mm; I: 1.8-3mm).

-   -   Gran1 (G1): 100 P of PP1, 20 P of POM1, 1 P of talc, 0.5 P of        Wax1    -   Gran2 (G2): 100 P of PP1, 34 P of POM1, 2.8 P of talc, 0.5 P of        Wax1    -   Gran3 (G3): 100 P of PP1, 25 P of glass, 0.5 P of Wax1    -   Gran4 (G4): 100 P of PP1, 20 P of PA1, 1 P of talc, 0.5 P of        Wax1    -   Gran5 (G5): 100 P of PP1, 25 P of PVP1, 1.25 P of talc, 0.6 P of        Wax1    -   Gran6 (G6): 100 P of PP1, 20 P of talc, 0.5 P of Wax1    -   Gran7 (G7): 100 P of PP1, 20 P of PS1, 1 P of talc, 0.5 P of        Wax1        Foaming of the Granules

The table shows the bulk densities achieved and the proportions of opencells measured on these foam beads. The proportion of open cells wasdetermined on the foam beads in accordance with DIN ISO 4590.

Examples 1 to 4

In each case, 100 P of granules, 6.7 P of Antiblock1, 0.13 P of Surf1,230 P of water and the amount of n/i-butane indicated in the table wereintroduced into a pressure-tight stirred tank. The tank was then heatedto the impregnation temperature (IMT) shown in the table over the courseof about 55 minutes, and, when this temperature had been reached, thegranules were foamed by decompressing the contents to ambient pressure.

Examples 5 to 10

In each case, 100 P of granules, 8.75 P of Antiblock2, 0.33 P of Surf2,234 P of water and the amount of n/i-butane indicated in the table wereintroduced into a pressure-tight stirred tank. The tank was then heatedto the impregnation temperature (IMT) indicated in the table over thecourse of about 55 minutes, and, after this temperature had beenreached, the granules were foamed by decompressing the contents toambient pressure. TABLE Example Example Example Example Example ExampleExample Example Example Example 1 2 3 4 5 6 7 8 9 10 Granules G1 G1 G1G1 G2 G3 G4 G5 G6 G7 IMT [° C.] 129.0 129.0 128.7 127.6 128.0 127.5130.0 129.0 130.0 136.0 Blowing agent [P] 24 20 20 20 24 24 35 24 35 16Bulk density [g/l] 18 31 30 37 15 11 14 9 15 13 Open cells [%] 43 42 5165 80 59 79 65 10 9[P] = Parts by weight per 100 parts of PP minigranulesExamples 9 to 10 are comparative examples

1. Open-celled foam beads having a mean bead diameter of from 1 to 10 mmand a bulk density of from 5 to 200 g/l based on propylene polymers andhaving a proportion of open cells (in accordance with DIN ISO 4590) ofgreater than 40%.
 2. Open-celled foam beads as claimed in claim 1,wherein the propylene polymer is a homopolymer or copolymer of propylenewith up to 15% by weight of a monomer selected from the group consistingof ethylene and 1-butene and mixtures thereof.
 3. Open-celled foam beadsas claimed in claim 1, which have, in the DSC thermodiagram, at leastone high-temperature peak at a higher temperature than the melting peakof the propylene polymer employed.
 4. Open-celled foam beads as claimedin claim 1, wherein the mean cell diameter is from 0.01 to 0.5 mm. 5.Open-celled foam beads as claimed in claim 1, which have a content offrom 1 to 40% by weight of a cell opener.
 6. A process for theproduction of open-celled foam beads as claimed in claim 1 byimpregnating propylene polymer beads in suspension with a volatileblowing agent in a pressure container at elevated temperature andsubsequently decompressing the mixture, wherein the propylene polymerbeads comprise from 1 to 40% by weight of a cell opener.
 7. A process asclaimed in claim 6, wherein the blowing agent is an organic compoundhaving a boiling point of between −5 and 150° C.
 8. A process as claimedin claim 6, wherein the cell opener is a polar, water-insolublethermoplastic.
 9. A process as claimed in claim 6, wherein the cellopener is a needle-shaped inorganic solid.
 10. A process as claimed inclaim 6, wherein the cell opener is a water-soluble polymer.
 11. Anopen-celled foam molding produced by post-expansion and sintering of thefoam beads as claimed in claim
 1. 12. The process as claimed in claim 6,wherein said blowing agent is selected from the group consisting of C₄-to C₆-hydrocarbons and inorganic gases.
 13. The process as claimed inclaim 8, wherein said polar, water-insoluble thermoplastic is selectedfrom the group consisting of polyamide and polyoxymethylene.
 14. Theprocess as claimed in claim 9, wherein said inorganic solid is cut glasshaving a length of from 0.25 to 5 mm.
 15. The process as claimed inclaim 10, wherein said water-soluble polymer is selected from the groupconsisting of polyvinylpyrrolidone, polyvinyl acetate, and polyethyleneoxide.